Atmospheric Monitoring and Precise Spectroscopy of the HR 8799 Planets with SCExAO/CHARIS

TL;DR

This study used advanced spectroscopic techniques to observe the HR 8799 planets, setting new upper limits on their atmospheric variability and providing highly precise spectra that reveal differences in their atmospheric compositions and cloud structures.

Contribution

First detailed spectrophotometric variability analysis of HR 8799 planets with the SCExAO/CHARIS system, providing the most precise spectra and insights into their atmospheric properties.

Findings

No significant variability detected in HR 8799 c and d.

All three planets favor supersolar metallicity with ~1100 K temperature.

HR 8799 d shows a distinct spectrum from HR 8799 c, suggesting different cloud properties.

Abstract

The atmospheres of gas giant planets are thought to be inhomogeneous due to weather and patchy clouds. We present two full nights of coronagraphic observations of the HR 8799 planets using the CHARIS integral field spectrograph behind the SCExAO adaptive optics system on the Subaru Telescope to search for spectrophomometric variability. We did not detect significant variability signals, but placed the lowest variability upper limits for HR 8799 c and d. Based on injection-recovery tests, we expected to have a 50% chance to detect signals down to 10% H-band photometric variability for HR 8799 c and down to 30% H-band variability for HR 8799 d. We also investigated spectral variability and expected a 50% chance to recovery 20% variability in the H/K flux ratio for HR 8799 c. We combined all the data from the two nights to obtain some of the most precise spectra obtained for HR 8799 c, d, and e. Using a grid of cloudy radiative-convective-thermochemical equilibrium models, we found all three planets prefer supersolar metallicity with effective temperatures of ~1100 K. However, our high signal-to-noise spectra show that HR 8799 d has a distinct spectrum from HR 8799 c, possibly preferring more vertically extended and uniform clouds and indicating that the planets are not identical.